configurable study fields

There should be custom configurable fields for following tables:

• visits
• subject study
• subject

Consider following type of columns:

• boolean
• integer
• double
• text
• list of labels
• date
• file (access to file must be secured because it's sensitive data)

Additional options:

• consider field as obligatory
• field might be readonly (data will be only automatically imported)
• field might be unique in the study